Self-contained mechanical actuators, either of the jackscrew or hoist type, commonly include a drive motor, a drive shaft extending therefrom to a connection at the input side of a speed reduction gear train (e.g. 20:1 reduction ratio), and a load-bearing arrangement which is operated by the output side of the speed reduction gear train. Typical disclosures of such a self-contained mechanical actuator are presented in U.S. Pat. No. 3,559,499 and U.S. Pat. No. 3,587,796, the actuator described in these patents also including a self-locking transmission mechanism which acts to transmit rotation of the drive shaft to the gear reduction train and to provide a self-locking drag which is imposed on the drive shaft to stop rotation thereof when power from the drive motor is terminated, thereby acting as a brake. While such a self-locking transmission is satisfactory in operation, it generates noise which may be objectional, and it has a tendency to allow a slight amount of reverse drift of the loadbearing member before complete braking is obtained.
It has been proposed heretofore in U.S. Pat. No. 3,651,906 to employ a fixed housing having ball elements contained therein and biased against a clutch element to prevent temporarily any rotational movement of the clutch element upon startup of drive shaft rotation. However, because the ball element housing is fixed, a problem may be encountered in properly aligning the housing so that an even distribution of ball element pressure can be obtained. Moreover, and perhaps because of this potential uneven pressure distribution, the clutch element surface upon which the ball elements act has flat edges which act to increase the pressure of the ball element on the clutch element, such flat edges also acting to cause constant reciprocation of the ball elements in their respective slots whereby significant wear is encountered as well as a high noise level.
It has also been heretofore proposed in U.S. Pat. No. 3,096,453 to utilize biased ball elements in a driven shaft telescopically received in a drive shaft, the ball elements acting as a point contact frictional drive for the two shafts as well as a frictional drag on the drive shaft when the driven shaft is mechanically braked. However, this arrangement requires a relatively precise alignment of the two telescopically arranged shafts and a mechanical brake must be provided for the purpose of stopping the rotation of one of the shafts.
In substantial contrast, the mechanical actuator of the present invention includes a brake arrangement which is very simple and reliable, and which is self-aligning and easily mounted in place to impose the necessary braking force even on a high-speed drive shaft.